This invention relates generally to techniques for joining one material to a different material and more particularly to a method of bonding together dissimilar materials, which is highly suitable for fabricating transition insert material for use in, for example, liquefied natural gas (LNG) freighters.
As a transition insert material, a composite member formed by bonding together an aluminum alloy and a stainless steel is ordinarily used. Since the fabrication of such a composite member by a fusion welding method is impossible, it is quite natural to consider the application of a diffusion welding method as an alternative. However, because the surface oxide of aluminum is extremely resistant to joining, it is necessary to raise the diffusion welding temperature to a value immediately below the melting point of aluminum in the case of diffusion welding of aluminum and steel, and even in the diffusion welding mutually between aluminum base materials.
In the case wherein the welding temperature is raised in this manner in the bonding together of the dissimilar materials, aluminum and steel, a brittle intermetallic compound of Al-Fe base is formed at the juncture therebetween, and the formation of a joint having ample reliability cannot be achieved.
According to results of our tests, when the diffusion welding temperature is low in the case of Al-Fe diffusion welding, the bond strength is almost zero, and when the welding temperature is raised, the bond strength is low and deviates over a wide range.
For these reasons, transition inserts are being fabricated simply by explosion welding in spite of the excellent economy of diffusion welding. As a consequence, transition inserts are extremely high in price in the present state of the art. Moreover, explosion welding is disadvantageous in that the kinds of aluminum alloys which can be used are restricted, and the materials cannot be freely selected. Furthermore, since the thickness of a member which can be thus explosion welded is limited, transition insert material of the desired thickness cannot be fabricated in all cases, whereby the assembly welding work must be carried out at the work site with due consideration of the heat transmission to the joint interface of the explosion welded transition insert. This gives rise to a problem in work efficiency and productivity.